Generally, a stainless steel including Cr in a content of 12 to 15% in steel is generally referred to as 13% Cr steel. Such 13% Cr steel is excellent regarding the mechanical properties, such as the yield strength, the yield stress and the like, as well as regarding the heat resistance, so that the steel may be used, for instance, as a material for an oil well pipe.
In the process of manufacturing 13% Cr steel, a steel material is normally heat-treated at a high temperature and quenched, so that a scale layer having a double layer structure consisting of an inner layer scale and an outer layer scale are inevitably formed on the surfaces of the 13% Cr steel. The inner layer scale includes mainly the spinel type FeCr2O4 having a high weather resistance, FeO, Fe3O4,Fe2SiO4 and the like, and it has an excellent adhesivensess to the surface of the base material of the 13% Cr steel, whereas the outer layer scale includes Fe2O3, Fe3O4 and the like and it has a less adhesiveness.
Before shiping the 13% Cr steel, descaling was compulsively carried out by applying a treatment such as the acid pickling or the shot blast to the surface having the scale layer formed. In recent years, however, 13% Cr steel is increasingly shipped without any application of descaling process to the surface thereof in order to reduce both the number of processes and the production cost.
On the other hand, a high weather resistance is required for the 13% Cr steel and, therefore, rust preventive oil is conventionally applied to the surfaces of the steel. When, however, the 13% Cr steel having scales is shipped after applying such a rust preventive oil to the surface thereof, only the outer layer scale having less adhesion peels off, thereby causing the rust preventive oil to be removed from the surface, together with the outer layer scale. As a result, no sufficient weather resistance can be obtained.
Various methods for manufacturing such 13% Cr steel were investigated in order to obtain an excellent weather resistance.
It is well known that the state of the steel surface plays an essential role for enhancing the weather resistance. In order to improve the surface state of the 13% Cr steel, a method has been demonstrated wherein the base material is heated under an oxygen-free atmosphere and then quenched. It is also well known that the main components of the scale layer formed on the surface of the steel are iron oxides, and neither the 13% Cr steel is oxidized nor the scale layer is formed in the manufacturing method under such an oxidation-free atmosphere.
Since, therefore, no outer layer scale peels off, the application of the rust preventive oil to the steel surface provides a sufficient high weather resistance. In this method, however, an additional apparatus for producing the oxygen-free atmosphere is required. This causes both the installation cost and the running cost to be increased, and eventually the cost of manufacturing the 13% Cr steel is increased.
In another method for enhancing the weather resistance, the heating temperature is decreased in the quenching process. This method provides a decreased amount of the scales formed on the steel surface. However, there is no substantial improvement regarding the formation of the scale layer itself. As a result, this method also provides a similar problem in which the outer layer scale peels off in the shipment and the rust preventive oil applied to the steel surface is also removed, together with the outer layer scale.
On the other hand, it is assumed that, in order to prevent the outer layer scale from peeling off, the outer layer scale should be preferably removed beforehand. From this viewpoint, Japanese Patent Application Laid-open (Kokai) No. 11-302802 has proposed a method for manufacturing a martensitic stainless steel, wherein, after descaling, the quenching treatment and the tempering treatment are sequentially carried out. Japanese Patent Application Laid-open (Kokai) No. 11-302802 has further disclosed a method for manufacturing a martensitic stainless steel, wherein, after the quenching treatment, the descaling and the tempering treatments are sequentially carried out.
The martensitic stainless steel manufactured with anyone of these methods provides an excellent weather resistance, since the outer layer scale, which is apt to peel off, is completely removed. Nevertheless, an outer scale is again formed in the final process of tempering. Accordingly, in order to obtain a martensitic stainless steel having a high weather resistance, it is important to properly specify how the scale formed in the final process of tempering is treated.